Fluid Dynamics
by Starwind77
Summary: Charlie has more than just hugging on his mind when he chases Don into the koi pond. A new theory of fluid dynamics is postulated, and subsequently tested by the brothers...together. In the shower. Don x Charlie incest


**Title:** Fluid Dynamics  
**Rating:** R for implied sexual situations  
**Spoilers:** Bones of Contention (S2 E10)  
**Warnings:** Incest  
**Word Count:** 3,938  
**Description:** Charlie has more than just hugging on his mind when he chases Don into the koi pond. A new theory of fluid dynamics is postulated, and subsequently tested by the brothers…together. In the shower.  
**Author's Note:** I loved the ending of this episode to pieces 3 and I couldn't help but extrapolate it to...well, its logical conclusion in the mind of us slashers . Fun fact: The Navier-Stokes equations mentioned in the fic are a set of differential equations used to model fluid flow; the proof of their existence and smoothness in 3 dimensions is, in fact, one of the Millennium Prize Problems, along with the Riemann Hypothesis and our favorite P vs. NP.

_Due to FF's posting restrictions, some scenes were edited to comply with ratings guidelines. You may read the complete fic at my writing journal (see profile)._

Alan's grey Acura pulled out of the driveway with a smooth rumble, leaving his two sons to stare conflictedly after their father's fading form…and the boxes of memories piled up to the window in the trunk.

"You _really_ gonna talk?" Charlie asked after an uncomfortable silence.

"I bought a six-pack. If things go south, we can just watch the game." Don shrugged and glanced over at his brother.

A wry smile quirked Charlie's lips. "Wanna laugh? That last box of stuff dad packed into his car..." He shook his head at the image. "...was my stuff."

"No way!" Don broke into a grin.

"Yeah."

"Oh, no." He couldn't help laughing. The thought of their dad marching determinedly into Goodwill, only to unpack a box not of dresses, but stacks of textbooks on carbon dating, nearly brought tears to his eyes. He was definitely gonna have to stock up on the beer tonight.

Charlie rubbed his hands together, looking down in embarrassment. "I didn't have the heart to tell him," he admitted abashedly.

An affectionate glow warmed Don's eyes, and he gazed once more over the tree-lined streets. His brother was always like that, too kind and thoughtless for his own good, still worrying more about others than himself. Except when it came to math, Charlie tended to shy away from blunt confrontation, preferring to let white lies slip by if it would mask a pervading hurt, or shatter the safety of his home. In fact, after mom passed away, it was mostly…for him that dad kept the house, perhaps thinking that the lingering tenderness of Margaret's touch would draw Charlie out of his shell.

A sudden pang of guilt ran through Don. "Hey, y'know..." The words came out awkwardly. "I don't have a problem with hugging." He shifted, trying to make it sound offhand. "I mean, I don't."

"I don't have a problem with hugging either," Charlie agreed readily.

There was a pause, and then a mischievous grin split his face as he opened his arms wide towards his brother.

Don began backing up immediately.

"Get away..." He raised his hands up, chuckling. "Get off of me."

"Just a little," Charlie teased.

"Get off of me!"

Not to be deterred, his brother started circling playfully around the yard.

"Hey, you were the one who said - "

Charlie made a dive, just as Don got his fists up in a good-natured round of roughhousing.

"I'm gonna put your lights out! "

" - you didn't mind hugging."

"Get outta here!"

He twisted, slipping out of Charlie's grip on his waist, and skipped back a few steps with the other clinging tenaciously to his thigh.

"Ready to give up?"

"Not a chance."

His brother grinned up at him, planning another spring, and Don weaved back and forth daring him to try. At the next attack, he feinted to the left, ducked low – five years of playing basketball with Charlie taught him all the right tricks to play – narrowly avoiding the other's outstretched hands as he took the chase to the back of the house.

"You know, there're pursuit curves to describe where you're converging!" Charlie called as he raced after his brother.

"Yeah, you just try that, buddy," Don challenged, dodging easily behind some shrubbery, "and we'll see how well it does against my tactical evasion skills."

Laughing like kids, they chased each other around the garden, alternately grappling and teasing and shouting amiable insults that drifted upward on the summer breeze. Charlie's exuberant dashes matched Don's long, playful strides, as their voices threaded through leaves of maple and pine above. Years seemed to fall away with every step.

The warmth of the afternoon, the scent of fresh grass beneath their feet, mingled with the chatter of neighbors and songbirds to bring back a surge of nostalgia. For a minute, Don could forget about the trials of the past few years…decades, almost…and just allow himself to enjoy the excitement of the moment. Adrenaline pumped from silly games, not violence, his brother's eyes lit up by the thought of _him_; just the two of them, together. He suspected it was more than the rush of the chase that made his cheeks glow whenever Charlie took a tumble at his feet.

"Hah!"

Seeing Don's lapse in vigilance, Charlie threw himself in a full tackle at his brother, catching the other squarely around the chest.

"Okay, okay, you win!" Don raised his hands in defeat. His face ached from laughing. "I surrender, I give up!"

Nevertheless, he took a step back to steady himself, and at that second, Charlie shifted his leg, resulting in them both hitting the slippery patch of grass at the same time…and tripping handily straight back into the koi pond. There was a tremendous splash, a flurry of fleeing fish, as Don found himself momentarily doused in shivering cold water with Charlie on top of him. A still-clinging, flailing Charlie. He sputtered, grabbed at the nearest handhold (which just so happened to be the other's collar – his brother deserved a dunking too) and braced himself against the edge of the pond until he could gulp air without drowning.

Several moments passed, as they both gasped for breath. Don was keenly aware of the proximity of bare skin - Charlie's shirt having half ridden up his chest in the tussle - and his brother's weight straddling him from above.

"Alright, so?" he asked with all the nonchalance he could muster. "Now that you've got me, what're y'gonna do?"

Charlie was hardly generous.

"Take every advantage of my position."

Before Don could react, he landed a big, sloppy, thoroughly unbecoming kiss right on the other's lips.

"Hey, hey, hey! Augh, get off me, Chuck!" Don flipped his brother into the water and made a big show of wiping at his mouth, even as he couldn't stop teasing. "What, Amita never taught you how to kiss right?"

"No, I learned that from you." Charlie grinned innocently.

"Yeah, sure, and Larry gave you swimming lessons."

He was not a little amused when his little brother took the remark seriously.

"You know, that's not entirely untrue. There was that time at the 5th annual CalSci miniature sailboat competition…" Charlie tilted his head, thinking. He didn't seem to notice the smudge of dirt on his face. "Our boat veered off course, and – I told him the propellers would adjust for the surface wave effect, but he insisted on adding an underwater counterweight, which just so happened to be, well, his trousers…"

"Alright, forget it. That's not an image I need right now." Don shook his head and pushed himself up with a heave, taking care not to do anymore damage to his father's beloved water plants. Turning, he offered a hand to Charlie. "Come on, if you lie around in the pond any longer, you're gonna end up killing all the koi."

Charlie bounced up in a splash of water, unperturbed by his plastered clothes. "Hey, those koi survived baseball season with you! I seem to remember Tombrello disappearing for a month after you landed a hit right on his back."

"Oh, yeah? Well, that's nothing compared to the time you tried to prove that uh, Navy Chokes - "

"Navier-Stokes."

" - equation of yours by dumping a gallon of honey in the tank." It was slightly less than that, actually, thanks to a serendipitous intervention in the form of morning flapjacks, but Don wasn't about to mention his little escapade into thievery to his brother.

"That experiment was a careful study of turbulence flow in non-Newtonian fluids."

"Right, which is why your first words to dad were, _Oops!_" He chuckled as he pushed open the door to the house.

They continued bantering back and forth, dripping water across the foyer floor as they made their way inside. The cool blast of air-conditioning sent an involuntary shiver down Charlie's spine, and he cast around the living room for a change of clothes. Finding none, he grabbed a blanket from the couch instead and refocused his attention on repudiating his brother, who was busily scavenging for leftovers in the kitchen fridge.

"…and then you made me get rid of the rest for you!" Don looked up with half a bagel in his mouth.

"I didn't make you do anything. _You_ offered, in exchange for me not telling mom about the babysitter incident." For once, Charlie was the one to swipe the remaining food – toroidal arch – from Don.

"Wait a second…I don't remember this deal."

"Friday night? In the garage?" Charlie raised an eyebrow. "You were 'teaching' her air hockey?"

"I'm gonna need some photographic evidence there, buddy," Don said with a lopsided grin, and laid a hand on his brother's neck.

"I'll give it to you," Charlie replied, "soon as you admit to your end of the bargain." He swiveled his body so they were face to face, touching, sharing the warmth of each other's embrace.

The blanket slid from Charlie's shoulders to the ground to be replaced by Don's strong, steady arms around his waist, and he pressed unconsciously into his brother's hips, reveling in the wet jeans that clung to every inch of hard, sculpted muscle. As Don wound and unwound his thumb around Charlie's damp, silky curls, so Charlie played with the tightness of his belt loop, no little detail escaping those desire-darkened eyes.

"Okay, okay. Truce," Don finally proposed before their fooling around got out of hand. "How 'bout we settle on payback for the both of us, huh?"

"What did you have in mind?"

"Well, y'know…I'm sure we can think of something…" He let his hand slip tighter around Charlie's back, bringing their foreheads together. "What about hugging?"

Charlie walked his fingers up Don's neck. "What about kissing?" he teased.

"What about…" A seductive smile tugged at the corner of the other's lips. "We do a little bit of both?"

Abruptly, Charlie stopped his ribbing, realizing quite rightly the ramifications of the invitation. A worried look creased his brow.

"W-wait, dad's gonna be back in - "

"You know he'll end up spending twice the time finding the donation agency, and then realize he gave away your stuff on the way back." Don shrugged. His hand never stopped massaging the other's back. "But if you're worried, we can always go to my apartment."

The daredevil glint that lit up Charlie's eyes was every adrenaline junkie's wet dream. "No. Let's do it here." He leaned in, whispering, "In the master bathroom."

The grin that spread on his brother's face was worth the delayed response.

--o--

Hot water beat down from the shower head, sending wisps of steam up to mist the bathroom mirror. Charlie was first to strip off his wet button-up and jeans, toss his clothes into the hamper, and step into the fine, relaxing spray. Tension fell from his shoulders like an iron weight. He started soaping up his neck and chest, the bubbles tickling at his nose, all the while eyeing his brother impatiently.

"Shaving cream, conditioner..." Still half-dressed (alas, the better part), Don poked around in the clutter beneath the bathroom sink, searching for an item to suit their purposes. He pushed aside a bar of soap and stopped, blinking quizzically, as he pulled out a light peach bottle. "Since when did dad start buying hand lotion?"

"Since he started dating again." Charlie poked his head out from behind the shower curtain. "Try the _Thymes_. It has a friction coefficient of less than three thousandth."

Don gave his brother an incredulous look. "Don't tell me there's some kind of a…lubrication theory, Chuck." Nevertheless, he set the lotion on the edge of the tub and quickly kicked off his remaining clothes, joining Charlie in the shower.

"Would you believe me if I said there was?"

The hand that ran along his breast was light and inviting, hardly shy.

"No." Don leaned in with a smile. "But I might if you show me."

Charlie needed no more encouragement to capture his brother's lips in a heated kiss, the soap slipping from his fingers as he looped his arms around the other's neck. Hot prickles of water sprayed against his face and trailed down the curve of his cheekbones to mingle with tongue and saliva, bubbles of foam on his chest that clung to the hairs on his skin.

"Mmm." For his part, Don sucked on the other's lips greedily, baitingly, not quite meeting Charlie's mouth, even as his breath began to quicken in the dense, steamy air. He toyed with his brother's nipples playfully and watched for a reaction as they hardened against his thumb. The slight hiss of breath told him he was right on – Charlie was always sensitive to subtle movements there – but his advantage was short-lived, as Charlie dropped a hand to the small of his back, that curve of muscle so tightly wound, and began to massage it in slow, circular strokes.

The tilt of their heads finally locked together like two puzzle pieces, drawing them into a deep, open-mouthed kiss.

Charlie let his tongue battle with his brother's for several seconds before relinquishing the prize he knew the other wanted, access to the slippery interior of his mouth, their tongues twining together in tandem. Movement became defined by how well he could steal gasps of air, while keeping pace with the sway of lust and contention. A soft moan escaped him at Don's suggestive thrusts, and he slid a hand urgently up the other's inner thigh.

"Want you…"

Hearing this, Don reluctantly broke the kiss to string gentle love bites down the crook of the other's neck and collarbone. A small smirk twitched at his lips.

"Since when did you get so big, huh?" He whispered impishly in his little brother's ear, as he nudged the other's legs apart with one knee.

A light blush reddened Charlie's already heat-drenched cheeks, and he responded by shifting his gaze down Don's waist. "Look who's talking, bro."

The other's chuckle warmed his skin, clouding his eyes with wanton pleasure, as they moved against each other like two choreographed dancers, tangling and embracing and wrapping limb around limb in practiced movements only siblings turned lovers could know so well. A soft cry escaped Charlie, back arching into the slippery white tiles, and moments later Don followed as well with a hoarse moan that drew shivers down his spine, bent double over his brother's naked, wet body.

It seemed like an eternity before they separated, each panting heavily against the other, muscles slicked with a sheen of sweat and soap.

Charlie broke the silence first.

"That was..." A long pause followed the other's flushed, thoughtful look, and he finally said, "Anti-symmetric."

Don could only blink.

Chuckle.

Then –

Perhaps it was the heady fog of the shower, or the utter ease that pervaded his bones, but he found himself cracking up uncontrollably at the absolute absurdity of the comment, sexual innuendo nested in some kind of obscure – but no doubt factual – mathematical jargon. His brother grinned crookedly, then snickered, and they held each other like that, laughing together beneath the sluice of warm, soothing water.

--o--

"You know, your mother…was never the sentimental type." Alan spoke haltingly, his hand wrapped loose around an unopened beer bottle as he gazed through the golden globe of light illuminating the living room. Night had fallen before he'd returned to the house, a good deal less agitated and bearing a conciliatory box of pizza, which he'd set on the table beside Don's six-pack, making a hearty meal for three.

Don suspected he knew where his father had gone, but he said nothing. He and his brother simply sat listening in silence.

"She kept everything from work meticulously filed, but if it concerned personal odds and ends, she wouldn't have clutter around the house." Shaking his head, Alan paused. "That's why I kept everything in the garage."

"Yeah, mom always used to nag you to clean the place out," Charlie offered quietly, fiddling with the edge of his _Coors_ bottle label. He knew, too, where their father had stopped; it didn't take a combinatorics degree to figure out the supermarket lay adjacent to the flower shop, and across from where their mother rested.

"When it came down to…to the last days, I asked her if there was something she'd like to…take with her when she left." A forlorn smile crinkled Alan's face, as he contemplated the absurdity of the question. It was one of those things you didn't think about – wills were for giving away possessions, not directing arrangements at funeral homes – except in the context of dusty museums about ancient Egyptian pharaohs. "And you know what she said?" He looked at his two sons. "I don't care what you put me in, so long as it's not that ridiculous dress from Hannukah."

"Hah," Don gave a short laugh, and Charlie chuckled a little sadly, recalling the events that led up to that unfortunate mishap.

Barely in middle school, they were, and he still flush from the work on his first set theory paper. Somehow, writing an equation to pick the best holiday gift for mom seemed like a good idea at the time. It didn't help that Don kept inserting variables into his morphology discrimination algorithm, while snooping through their mother's old clothing catalogs. The end result never quite looked right to either of them (even sans fashion sense), but Margaret had simply laughed at the yellow dreidel prints and kissed them both on the cheek for their doting efforts.

Alan gazed long and deep at the photograph of his wife on the wall.

"But she always understood what was really important," he said, a distant look in his eyes. "A family. A home. Those people you put your heart and soul into."

"Dad…"

Not trusting himself to say more, Charlie laid a hand on his father's shoulder, and Don too reached over with a comforting hand, words unspoken laden in that gesture that connected the three of them together. Alan clutched his younger son's arm tightly, gazing fiercely into both of their eyes through a tremulous haze.

"You boys meant so much to her," he whispered thickly.

They held each other like that for several moments, as much out of necessity as comfort, until Alan finally loosened his grip and blinked embarrassedly at the moisture clouding his sight. Nevertheless, the brief lapse in emotion had lifted a tremendous ache in his chest – one he'd kept hidden away till today. He cleared his throat and popped open a bottle of beer, switching subjects rapidly.

"By the way, Charlie. I brought your box back."

"Thanks." Charlie hesitated, cautious. "Did you…"

"Lady at the agency said they didn't take craft supplies."

"Wait, what was in it?" Don asked, puzzled.

"Your elementary school work! You know." Alan looked at his two blank-faced sons. "All that archaeology dig stuff from fifth grade? The jars and shovels and history textbooks." He waved a hand in the air, as if it should be obvious. "I'm surprised you needed it for the case."

"Oh, yeah…" Recognition dawned on Charlie's face, and he shook a finger at Don, recalling. "I remember now. That time we had that class project - "

"The time capsule, right?" Don's eyes brightened at the memory – one of the few homework assignments he'd outdid his brother on. "You had to pick five things to leave for people in the future."

"And Charlie chose five of his math equations."

"Hey, you know, those equations weren't arbitrary. I picked them to encompass _all_ aspects of material behavior." Seeing the surrounding skepticism, Charlie frowned and set his beer down with a _clunk_, revving up into full lecture mode. "The Fundamental Theorem of Calculus is used in change rate analysis, optimization of nonlinear systems like the firing of neurons in your brain or the swing of that – that pendulum in our grandfather clock." He cast about for some tool to demonstrate his point, but failing that, waved a hand in the direction of the entrance. "Maxwell's equations describe completely the phenomena of electromagnetic waves. Your electricity and lights that make this house run. And Euler's identity," Charlie leaned in, fairly glowing with intensity, "such a simple expression to the untrained eye, links three of the most important constants in Euclidean geometry, transcendental number theory, and the complex plane."

You could never quite walk away from one of Charlie's talks without feeling entranced (as Terry well knew), even when you couldn't follow half of what he said.

"Alright, we believe you, Charlie," Don said to placate his brother. He pushed the half-empty bottle back into the other's hand. "But see, the problem with that approach is, wouldn't you expect future mathematicians to know all that already?"

Charlie halted mid-lecture, and realized he'd overlooked the obvious. "Well…yeah." He trailed off weakly. "I'm just leaving an intellectual history," he pointed out in his defense.

"What about you, Donnie, you put all your baseball cards and little league medals in." Their father redirected the questioning.

"Aw, you can't go wrong with _baseball_." Grinning goofily, Don took a swig from his beer bottle. "It's an all-American sport!"

"How do you know America will even be here in a hundred years?"

"Well, I mean, I don't," He shrugged, vacillating. "But you haveta expect the Chinese're gonna pick up on it someday."

That brought a chuckle to everyone's lips, and they reclined contentedly around the living room table, ensconced in the warm fabric of security. Don felt his eyelids grow heavy as the evening waned – the beer always brought a nice buzz to his brain, swimming with the events of the day – and he could see that Charlie too was exhausted, head drooping against the sofa. A pillow would be nice, he contemplated idly. Two even. Perhaps they'd share their old bed tonight…

"You know, there was one thing you two put in that was exactly the same," Alan said, interrupting his reverie. "Do you remember what it was?"

Don wrinkled his forehead, mind drifting over chalk-lined baseballs and doodled-over math textbooks, old albums of photographs.

"Yeah…yeah," He replied thoughtfully, a nostalgic tinge in his eyes. "It was a picture of you. And mom."

"Of all of us," Charlie added in a soft voice.

Alan leaned in, taking both of his sons by the shoulders. "Because no matter what happens," he said, looking firmly at each of them, "family will always be together."

He clapped them on the back and got up, plate and empty pizza box in hand.

"Anyway, I'm going up to bed. Another day tomorrow, big project over at the shopping center they want me to consult on. Lots of urban planning."

Alan paused on his way to the kitchen.

"By the way, did one of you boys take my hand lotion?"

Mid-swallow, Charlie choked on a mouthful of _Coors_, and Don had to bury his smile in his beer bottle or risk losing it at the image of their father finding his _Thymes_ half-empty on the bathtub floor.

"Something the matter?" Alan glanced at his sons, puzzled.

"Nothing, dad. Have a good night," Charlie managed between a few hoarse coughs.

"I'll uh...I'll be up a little later to do the dishes."

They sat in frozen silence until their father's footsteps faded up the stairs, safely out of hearing, before a broad grin spread around the table.

"You _really_ gonna study the fluid dynamics of dishwasher flow?" Charlie asked his brother with more than a hint of skepticism in his voice.

Don merely chuckled.

"Only if you're there to help me, buddy," he winked.


End file.
